1. Field of the Invention
The present invention relates to a thermosensitive recording material on which colored images are formed by heating. More particularly, the present invention relates to a thermosensitive recording material capable of forming thereon colored images having a high resistance to fading and thus exhibiting a high persistency during storage thereof for a long time.
The thermosensitive recording material of the present invention is able to record thereon colored images exhibiting an excellent resistance to moisture, heat, oily and fatty substances, and plasticizers, and thus has a superior persistency when stored for a long time and therefore is useful as colored image-recording sheets, sheets for use in facsimiles, word processors, CRT image printers and cash dispensers, as passenger tickets, commuter passes, labels such as POS labels, cards such as prepaid cards, and as transit passes.
2. Description of the Related Arts
It is known that a conventional thermosensitive recording material comprises a supporting substrate, for example, a paper sheet, synthetic paper sheet, or plastic resin film and a thermosensitive colored image-forming layer formed on a surface of the supporting substrate and comprising an electron-donative dye precursor, for example, a leuco basic dye, an electron-acceptive color-developing agent consisting of an organic acid substance, for example, a phenol compound, and a binder. When the thermosensitive colored image-forming layer is heated imagewise, colored images are recorded thereon by a reaction of the dye precursor with the color-developing agent.
This type of thermosensitive recording material is disclosed in Japanese Examined Patent Publication Nos. 43-4,160 and 45-14,039 and Japanese Unexamined Patent Publication No. 48-27,736, and is widely employed in practice.
Namely, the thermosensitive recording material is advantageous in that colored images can be easily formed by heating alone, and the recording apparatus can be made compact and small in size, has a relatively low cost, and can be easily maintained. Therefore, the thermosensitive recording material is appreciated as a useful information-recording material for recording outputs of printers used with, for example, computers, facsimile machines, automatic ticket-vending machines, scientific measurement recorders, and CRT medical measurement recorders.
Nevertheless, the conventional dye-forming type thermosensitive recording materials in which the thermosensitive colored image-forming layer comprises a conventional color-developing agent together with the dye precursor and the binder is disadvantageous in that the resultant colored images fade with the lapse of time. Presumably due to a reversible reaction of the dye recursor with the color-developing agent. This fading of the colored images is accelerated by exposure to light, high temperatures, and high humidity and is specifically promoted by contact with an oily or fatty substance or a plasticizer, to an extent such that the faded images cannot be recognized.
Many attempts have been made to retard or inhibit the fading of the colored images formed on a conventional thermosensitive colored image-forming layer containing a substantially colorless dye precursor comprising a lactone ring compound.
For example, Japanese Unexamined Patent Publication Nos. 60-78,782, 59-167,292, 59-114,096 and 59-93,387 disclose a thermosensitive colored image-forming layer containing a phenolic antioxidant.
Japanese Unexamined Patent Publication No. 56-146,794 discloses a protective layer formed from a hydrophobic polymeric compound emulsion on a thermosensitive colored image-forming layer.
Japanese Unexamined Patent Publication No. 58-199,189 discloses formation of both an intermediate layer and a top layer on a thermosensitive colored image-forming layer, and the former being formed from a water-soluble polymeric compound solution or a hydrophobic polymeric compound emulsion and the latter being formed from a solvent-soluble hydrophobic polymer on the intermediate layer.
Japanese Unexamined Patent Publication No. 62-164,579 and No. 60-219,088 disclose a thermosensitive colored image-forming layer containing an additive consisting of an epoxy compound and/or an aziridine compound, which effectively inhibits the fading of the colored images.
In the thermosensitive colored image-forming layer containing the phenolic antioxidant, the resultant colored images exhibit a higher resistance to heat and moisture to a certain extend compared to the colored images formed on a convention colored image-forming layer free from the phenolic antioxidant, but the improvement effect of the phenolic antioxidant is not satisfactorily light. Also, the phenolic antioxidant does not have a capability of enhancing the resistance of the colored images to the oily or fatty substances, for example, salad oil, and plasticizers, for example, dioctyl phthalate. The resistance of the colored images to oily or fatty substance or a plasticizer is determined in such a manner that the colored images are brought into contact with an oily or fatty substance, for example, a salad oil or a plasticizer, and left in contact therewith for a predetermined time, and then a retention of the color density of the tested colored images is measured in comparison with an initial color density thereof.
When the protective layer or the intermediate and surface layers are formed on the thermosensitive colored image-forming layer, the resultant colored images exhibit a significantly enhanced storage persistency when the salad oil or the dioctyl phthalate is brought into contact with the colored image-forming surface of the recording material. Nevertheless, when the salad oil or the dioctyl phthalate is brought into contact with an edge face of the recording material, it penetrates into the inside of the recording material and causes a complete fading of the colored images. Therefore, the provision of the protecting layer or the intermediate and surface layer cannot completely eliminate the undesirable color-fading of the images.
The addition of the epoxy compound and/or aziridine compound to the colored image-forming layer is highly effective to inhibit the fading phenomenon. Nevertheless, this not totally appreciated, because it takes a long time to stabilize the colored images formed on the colored image-forming layer by a heat-recording operation, and therefore, if an oily or fatty substance, for example, salad oil, or a plasticizer, for example, dioctyl phthalate, is brought into contact with the colored image-forming layer immediately after the heat-recording operation, the resultant colored images fade to a great extent.